1. Field of the Invention
This invention relates to the field of hemodialysis, and, in particular, the invention relates to sterilizable dialysis systems using either coil or plate type dialyzers.
2. Description of the Prior Art
Hemodialysis systems are used for blood purification when a patient's kidneys no longer perform adequately or have been surgically removed. In hemodialysis, the patient's blood is circulated on one side of a large surface area membrane having microscopic pores through which waste products from the blood may pass but which are too small to permit passage of essential blood components. Opposite the blood side of the membrane is an isotonic fluid which is circulated to remove the waste products by dialysis. The salt or mineral concentration of the dialysate solution determines the rate and character of absorption of minerals from the blood. A pressure differential across the membrane controls water removal from the blood by reverse osmosis. Numerous critical parameters must be carefully monitored and adjusted to avoid trauma to the patient.
One of those factors is the specific concentration of dialysate solution. If the solution is too weak, excess minerals as well as uremic wastes may be extracted from the blood and the blood cells may be damaged. If the concentration is too strong, the salts may become absorbed by the blood with a resulting toxic effect. However, the cost of the concentrate itself is a significant factor in the total cost of dialysis.
Blood temperature and pressure must also be carefully maintained. Blood temperature maintenance requires precise monitoring of the dialysate temperature inasmuch as the dialysate is in heat exchange relationship to the blood as it passes across the membranes. Pressure is affected by the passage of blood through the dialyzer and depends upon the type of dialyzer and blood pump if used. Water removal from the blood is dependent on the pressure differential across the dialyzer membranes. It is vital that the dialyzer not become clogged, either with air or impurities which would prevent its functioning in transferring uremic wastes to the dialysate solution.
Apparatus which is used in connection with more than one patient requires sterilization. One difficult problem associated with use of medical equipment in the past has been the elimination of microbial organisms such as type B hepatitis. Sterilization in the past has usually been effected by hot water at about 85.degree. C. or by the use of formalin or sodium hypochlorite solution. Neither of these sterilization techniques is sufficiently effective to eliminate type B viral hepatitis. Because of their relatively large size, most dialysis systems are impractical to sterilize by autoclaving techniques.
Two main types of dialyzers are in current use. One is the so-called coil type artificial kidney or dialyzer which consists of a single tubular membrane which has been flattened considerably to provide high and efficient surface area for osmotic transfer between the blood and the dialysate solution. Since the blood must travel a considerable length through the elongated coil dialyzer, a blood pump is required. A substantial flow rate about the outside of the dialyzer kidney provides efficient dialysis. Proportional control of the dialysate has not been employed in this type of system. Typically, a number of the coil-type dialyzers are fed in parallel from a large central mixing station.
Parallel plate dialyzers utilize a multiple membrane stack between the membranes of which blood and dialysate flow in adjacent passageways. Since blood travels a shorter distance across a greater total cross-sectional area than with a coil dialyzer, blood trauma is less likely to occur and the shorter distance often requires less in the way of pumping pressure requirements. However, when a parallel flow dialyzer is used a negative pressure system is required to pull dialysate through the dialyzer. Principally because of such factors, proportional control of dialysate has been employed, but recirculation has not been, in parallel flow systems.
In the past, economic factors have strongly influenced the use of hemodialysis. One high cost factor is the central mixing room used for coil dialysis. The central mixing room typically requires 1000 square feet of hospital space for the mixing of dialysate solution. Two central processors are required for adequate reliability. Should a central processor become contaminated it must be completely shut down for sterilization. Solutions of sodium hypochlorite or formalin are then used to wash the system. Unfortunately, these chemicals also tend to leave residues which have a somewhat toxic effect on the patient. Typically twenty patients will be delivered dialysate solution from the same central mixing room unit. Extensive plumbing is required to transfer the dialysate solution from the central mixing room to the individual rooms of the patients or to the various beds of the ward. Yet each patient still requires an individual canister of dialysate solution.
Central delivery systems are not generally used with parallel flow dialyzers because this type of dialyzer requires an individual suction or proportioning delivery system. In the past, individual proportioning delivery systems have included servo controls to deliver dialysate within predetermined concentration limits. However, continuous uniform levels of concentration are difficult to achieve since the water and concentrate flows are pumped by separate positive flow devices. Though the long term average of the dialysate solution may not change, the short term proportion may change considerably as the pumping action proceeds. Since each of the separate flow devices have 100% control authority over the separate fluids prior to mixing, a failure of either one results in an immediate corresponding error. Since the delivery system is coupled almost directly to the parallel flow dialyzer minor variations during the cycle of the delivery system are seen at the membranes. These variations may be sufficient to alter the blood conditions to such an extent as to throw the patient into shock if not properly monitored.
Thus the coil and parallel dialyzers each have separate special considerations which must be met for their safe utilization. Economics of hospital operations are such that the dialysis systems in use must be transferred at various times to different patients, yet different patients require the use of different types of dialyzers. Thus, numerous problems are still present in providing safe hemodialysis to patients at moderate costs.